In a variety of low-temperature gas processes, e.g. air rectification by the LINDE-FRANKL process and, more generally, in the separation of gases by pressurization and liquefaction, or in the liquefaction of low-boiling gases, such as helium, it is frequently necessary to expand high-pressure gases to lower pressures or to compress low-pressure gases to higher pressures.
Expansion for instance can be carried out, as described in chapter 12, pages 29 ff. of Perry's Chemical Engineer's Handbook, McGraw-Hill Book Company, New York, 1963, in turbine-type machines or piston-type machines, the invention being concerned with machines of the latter type. The work expansion of a gas is usually carried out against a load which can be, for example, a generator of electrical energy.
It is known to provide electromagnetically controlled valves for the cylinders of such low-temperature expansion machines to admit the high-pressure gas and to discharge the low-pressure gas, and to time the operation of the valve by cam-operated switches which are mechanically opened and closed by cams carried by the crankshaft of the machine.
More specifically, from German Pat. No. 1,217,982, for example, it is known to provide a valve control system for the gas cycling of the cylinder of a piston-type expansion machine by electromagnetically actuated inlet and outlet valves.
Each of these valves has a magnetic armature which cooperates with an electromagnetic coil so that, upon energization, the force of a spring biasing the valve member into a closing position can be overcome and the respective valve opened.
The circuit used in this prior-art system comprises a pair of mechanically operated switches connected in series with a current supply source.
The valve itself is a substantially axially symmetrical structure with respect to the axis of the magnetic coil and uses an armature of disk or plate shape which is disposed on one side of the coil and can simultaneously form the valve plate.
The coil is enclosed in a field-determining yoke or stator which is composed of iron and which, upon magnetic deenergization, forms a comparatively wide air gap which can intercept or provide a significant obstruction to the magnetic field lines upon reenergization of the coil.
When the coil is not energized and the valve is held closed by the spring, the large magnetic resistance resulting primarily from this air gap mandates a high energization current to attract the armature by overcoming the magnetic resistance as well as the spring force.
In the open condition of the valve, the air gap is practically eliminated and, because of the magnetic remanence of the yoke, remains attracted even after the current supply of the coil is cut off so that valve closing is delayed.
The energizing circuit for the prior-art system comprises the current source connected in parallel to a condenser and to the coil, the switches being provided in one of the connections between the condenser and the coil while an ohmic resistor is provided in the other connection.
When the switch contacts are opened, the condenser is charged through a charging resistor and, upon closure of the switches by the cams, the capacitor discharges through the coil to supplement the current drawn from the source and therefore provide the high initial current surge required to overcome the magnetic resistance, the spring force and the inertia of the valve plate.
While the system of the German patent has been found to be a vast improvement over valve systems which are mechanically linked to the crankshaft, it has some significant disadvantages. For instance, the high magnetic resistance prevents high-speed opening of the valve while the magnetic remanence prevents high-speed closing.
Wear of the mechanical switch system requires frequent replacement thereof and leads eventually to timing problems.
Low-temperature piston machines for the purposes described should also have relatively small heat capacities and thus should also have small dimensions and be capable of operating at high frequencies, a requirement which cannot be adequately fulfilled by the earlier systems.